fix: key cursor telemetry batches by recordingId for safe discard

discardLatestPending() popped whichever batch happened to be at the
back of the queue. With a Stop → Record → Discard sequence, the
pending queue can have recording B's batch sitting in front of A's by
the time A's finalize callback resolves (because finalizeRecording
awaits fixWebmDuration), so the discard targets the wrong recording.

Tag each completed batch with the recording id supplied at
startSession() time and replace discardLatestPending() with
discardBatch(recordingId). takeNextBatch() now returns the full
{recordingId, samples} shape so prependBatch() can re-queue it on
write-failure without losing the id. The renderer already owns a
stable recordingId (Date.now() in useScreenRecorder) and the IPC
surface threads it through set-recording-state and
discard-cursor-telemetry.

Adds a regression test that mirrors FabLrc's scenario in PR #457:
two recordings finalize, A is discarded after B has already been
queued, and the buffer must drop A while keeping B intact.

src/hooks/useScreenRecorder.ts

@@ -225,7 +225,7 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 				try {
 					const screenBlob = await activeScreenRecorder.recordedBlobPromise;
 					if (discardRecordingId.current === activeRecordingId) {
-						window.electronAPI?.discardCursorTelemetry();
+						window.electronAPI?.discardCursorTelemetry(activeRecordingId);
 						return;
 					}
 					if (screenBlob.size === 0) {
@@ -554,7 +554,7 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 			setRecording(true);
 			setPaused(false);
 			setElapsedSeconds(0);
-			window.electronAPI?.setRecordingState(true);
+			window.electronAPI?.setRecordingState(true, recordingId.current);
 
 			const activeScreenRecorder = screenRecorder.current;
 			const activeWebcamRecorder = webcamRecorder.current;